This invention relates to an electric screw driver, and particularly to improvements relating to a clutch assembly of an electric screw driver.
Clutch mechanisms of electric screw drivers generally include an upper clutch member having a bottom disc portion with engaging projections to engage with engaging projections provided on a disc portion of a lower clutch member. An example of an electric screw driver is shown in FIG. 1, wherein numerals 11, 12, 13, 14, and 15 respectively represent a disc portion of an upper clutch member, an engaging projection 12 of the clutch member, a lower clutch member, a disc portion of the lower clutch member and projections of the lower clutch member. The lower clutch member is retained by a bearing housing 18 which is biased upward by a coil spring 17 so that the lower clutch member engages with the upper clutch member. A motor 10 is provided to impart a rotational motion to the screw driver head 16 through the clutch mechanism. In common practice, when a screw turned by the screw driver becomes tightly set in the object into which it is being driven, the head 16 cannot be turned further due to the immobilized screw. Accordingly, the lower clutch member is restricted from rotation. However, the upper clutch member rotates continuously so that the projection 12 intermittently passes the protrusions 15 by pushing them slightly downward against the torsion adjusting coil spring 17. Usually, impacts are directly imparted to the disc portions of the clutch member when the protrusions 15 are moved upward again by the coil spring 17 each time after they are pushed downward, thereby rapidly degrading the performance of the clutch members.